The present invention relates generally to a method or an apparatus for segregating or classifying particles. The invention relates particularly, though not exclusively, to a fluidized multistage lamellae classifier, particle fractionator or reflux classifier, being designed for segregating particles according to size or density.
In many industrial processes it is necessary to classify particles according to their size, and sometimes according to their density. For example, in mineral processing, screens, cyclones, and elutriators are often used to sort particles prior to downstream beneficiation. Classification may proceed either in a wet or a dry state. Although the aim may be to separate the particles at 2 specific size, there is usually a high degree of so-called misplaced material, with a portion of coarse particles reporting with the fine particle stream. An xe2x80x9cSxe2x80x9d shape partition curve provides a measure of the probability of a given particle size reporting to a given stream, and hence the efficiency of the separation.
In principle, sieves should provide the perfect separation, given that a particle will only pass through sieve if it is smaller than the openings. However, if the particles are not given sufficient time on the sieve a poor separation will be achieved. Relatively fine particles, less than 45 xcexcm in diameter, readily adhere to other particles, and are therefore difficult to separate using sieves. Sieves also tend to become blinded by particles which are similar in size to the openings, and operate poorly when particles are fed on a continuous basis.
Elutriators separate particles according to their settling velocity. If the particles are of the same density, then the separation proceeds in accordance with the particle size. A liquid passes up through the vessel at a specific velocity, carrying slower settling particles to the top, thus allowing faster moving particles to be withdrawn from near the base of the vessel. However, elutriators fail to provide satisfactory throughputs, especially when the separation size is relatively small. On the other hand cyclones provide remarkably high throughputs although their efficiency is arguably poorer, and the separation size more difficult to control.
Inclined classifier have the potential to offer satisfactory throughputs, and efficient separations. Earlier this century Boycott (1920) found that the effective rate of sedimentation could be increased significantly by inclining a vessel. Lamellae thickeners, which are gravity settlers containing parallel plates, are now used in solid liquid separation. However with these known systems there is no attempt to classify particles, only to remove all particles from much of the liquid. Basically the solids settle onto the plates once only and move downwards as a concentrate. In laboratories, inclined devices have also been used to classify particles according to both size and density.
According to one aspect of the present invention there is provided a classifier for segregating particles by size or density, said classifier comprising:
a fluidization chamber adapted to contain a fluidized bed;
fluidization means adapted to provide a flow of fluidization fluid into the fluidization chamber; and
one or mote inclined plates mounted within the fluidization chamber positioned such that in use, particles elutriated by the fluidization fluid within the chamber are caused to be segregated above or below the plates according to their size or density.
Preferably, said one or more inclined plates comprises at least one array of inclined plates.
Preferably, said array of inclined plates comprises an array parallel equally spaced plates.
Preferably, said array of plates extends substantially horizontally across said fluidization chamber.
Preferably, two or more arrays of inclined plates are provided, each array being vertically spaced from the or each other array, and dividing the fluidization chamber into zones.
Preferably, the length of each plate in an array, the angle of inclination of the plates, and the spacing between plates in that array are selected to enable particles of a predetermined size or density to pass through the array when elutriated at a predetermined rate by the fluidization fluid, while inhibiting particles of greater size or density from passing through the array.
In one form of the invention a feed fluid incorporating particles to be classified is fed into the fluidization chamber between two said arrays of inclined plates.
In another form of the invention the particles are fed into the fluidization chamber with the fluidization fluid.
According to another aspect of the present invention there is provided a method of classifying particles by size or density, said method comprising the steps of:
providing a fluidized bed within a fluidization chamber in which is positioned one or more inclined plates;
feeding the particles into the fluidized bed; and
withdrawing particles from the chamber at one or more predetermined locations.
Preferably, the chamber is provided with two or more arrays of said inclined plates, each array being vertically spaced from the or each other array thereby dividing the fluidization chamber into zones, and wherein the withdrawal of particles from the chamber comprises withdrawal from a selected one or more of said zones.
Preferably, said fluidized bed is provided with fluidization fluid at a predetermined rate so as to achieve desired separation of sizes or densities above and below said inclined plates in combination with selected sizes, inclination and spacings of said inclined plates.
It is understood that increased particle segregation is largely produced through the rise of the inclined plates and efficient classification achieved by the reflux effect of the fluidization fluid whereby said fluid can repeatedly fluidized into the plates particles of a certain size and/or density.
Generally the particles and vast majority of the fluidization fluid are introduced to the fluidization chamber as separate streams. Alternatively the particles are incorporated in the fluidization fluid which may for example be in the form of a slurry.
Each array of parallel, equally spaced plates forms a lamellae which acts as an inclined hydraulic screen causing the particles of different size or density ranges to locate into different zones within the chamber.
Typically there are provided a plurality of lamellae within the housing or fluidization chamber. In one embodiment the plates of adjacent lamellae stages are, in the direction of flow of the fluidization fluid, oriented differently whereby progressively slower settling particles pass each adjacent stage. For example, the plates may be longer, closer together and/or less steep in order to achieve this effect. In another embodiment the configuration of plates of adjacent stages is substantially the same for greater refinement of the separation process. It should be appreciated that other permutations of lamellae plate sequences and arrangements are within the scope of the present invention.
Typically, there is provided an uppermost lamellae designed to prevent particles from escaping the fluidization chamber together with much of the fluidization fluid. Thus, relatively fine particles can be removed or classified in a more concentrated form from beneath the uppermost lamellae.
It is believed that the system has the advantage of self-control. An increase in the feed rate does not require a change in the underflow rate to preserve the separation condition. Keeping the underflow rate constant will simply cause the suspension concentration to rise, which automatically changes the underflow solids rate to the right steady state level. The change in hindered settling will cause a slight change in the separation size, but this change will be substantially less than the change in the feed rate. The separation size for a given device is altered simply by changing the fluidization velocity.
A further advantage is the ease of underflow removal because of the high rate of fluidization made possible by the control achieved by the lamellae. Hence all particles (including those in the underflow) tend to be in a state of suspension.
Preferably the fluidization chamber or classifier housing is elongate and shaped generally square or cylindrical in cross section with the stages of lamellae plates spaced longitudinally along its length. More preferably the fluidization chamber or classifier housing is oriented upright with the fluidization fluid flowing generally upward. The cross sectional area of the chamber may vary from one stage to the next to alter the relative separation size.
Generally the plates are inclined. Additionally or alternatively the fluidization chamber may be inclined, or parts of the chamber may be inclined to follow the overall shape of the various stages of lamellae plates.
Advantageously, the step of passing the fluidization fluid through the classifier can involve alteration of the fluidization rate to promote segregation of the particles largely on the basis of density.
In one form of the invention the method or apparatus for segregating or classifying particles is performed as a batch operation. Alternatively the method or apparatus is operated in a continuous mode whereby the particles are continuously or intermittenly added to the fluidization chamber or the classifier housing. In this case, the feed is separated into an underflow stream and, at least an overflow stream.
Generally the fluidization fluid is water or another liquid. Alternatively the fluidization fluid is a gas. The particles may be provided dry, or wet as a slurry.
The term xe2x80x9cparticlesxe2x80x9d is to be understood to include solids, liquid droplets, and air or other gas bubble.
Suspension removed from specific zones between sets of parallel plates will contain particles of a narrow size range. The system can be operated as a particle fractionator in the batch or continuous mode, separating a feed into multiple size fractions.
In order to achieve a better understanding of the nature of the present invention several preferred embodiments of an apparatus for and a method of segregation or classifying particles will now be described, by way of example only, with referance to the accompanying drawings in which: